Killcams are only part of the story. A lot of people have spawns memorized in certain modes (FFA, especially) and can rush from spawn to spawn mowing people down. No wall hack. *Note: I don't do this. I have read it is fairly common, however.
Also, some people pre-fire or pre-ADS around corners, especially in high-traffic areas, which can seem like a, "How did he know I was there?" moment, but really are just strategy and some luck that you happened to be there.
For example, I was playing Outpost and ran down the ramp in the middle of the map. As I neared the bottom, over the top railing of the walkway heading to C flag, I saw someone's head poking over as they ran towards my end of the walkway and then I lost sight of them due to the camera angle. I quickly ADSed and "followed" where I thought the person was running behind the wall and, sure enough, killed them the second he ran out from behind the wall.
It was the final kill. You know what it showed the room? It missed the first part where you could see his head. All anyone in the lobby saw was me ADS and "follow" along the wall, shooting the second he came out the other side. To me, I could understand someone viewing that as a "wall hack" or perma-radar or whatever, but it was just circumstance. (No one in the room said anything about it, but I thought it was an interestingly coincidental killcam).
Personally, in all of my play-time, I have never legitimately suspected another player of cheating on the PS3. But, that's just my two cents, anyway.
